1. Field of the Invention
The invention relates to apparatus and methods for folding and sealing floppy disc envelopes.
2. Description of the Prior Art
Foppy discs are flexible annular recording discs which are widely used for off-line data storage in data processing systems due to their lightweight, relatively low cost, and convenience of use. Floppy discs, which are described in more detail in a co-pending application entitled "Improved Method and Apparatus for Burnishing Floppy Discs", Ser. No. 885,625, filed on Mar. 13, 1978, invented by the present inventor and incorporated herein by reference, are permanently sealed in floppy disc envelopes, hereinafter referred to as simply envelopes, prior to use. The envelopes are formed from sheets of flexible polyvinyl chloride. A floppy disc envelope has aligned center or drive holes which permit a floppy disc transport system to engage the floppy disc enclosed within the envelope and rotate the floppy disc between layers of a low friction liner with which the envelope is lined. The envelope also includes slots which permit electro-optical synchronization of the floppy disc transport system with rotation of the floppy disc and also permits magnetic read/write heads of the floppy disc machine to contact a rotating floppy disc.
The floppy disc envelopes are formed from precut sheets of polyvinyl chloride which include the necessary center holes, alignment holes, and magnetic head slots. Half of a pre-cut sheet is rectangular. The other half is identical, except that it includes three flaps which extend from its three outer edges. The low friction liner is attached to the inner surface of the sheet prior to pre-cutting it. A center fold having a predetermined fold radius is formed by folding each half of the sheet against the other half. The side flaps are then folded around the edges of the opposite half of the sheet and are sealed thereto. A floppy disc is then inserted between the low friction liner layer attached to the inner surface of the two halves or sectitons of the sheet and the end flap is then folded to complete assembly of a floppy disc and envelope unit, hereinafter referred to as a floppy disc unit.
In the past, the known techniques for folding and sealing the envelopes have been problematic. The four required folds must be very precise in order that torque specifications for the floppy disc unit remains within predetermined specifications. If the folds are too "sharp", the pressure of the low friction liner against an enclosed rotating floppy disc will increase the torque required by the floppy disc drive system. Similarly, warpage of the completed floppy disc unit must be kept within tight tolerances established by users of floppy disc systems. Unfortunately, it has been difficult to provide the required precise folds in polyvinyl chloride sheets necessary to avoid unacceptable warpage of the completed floppy disc units to the extent required to obtain high manufacturing yields. In the past it has been necessary to heat the polyvinyl chloride sheets at the fold locations prior to bending of the sheets. One known way of accomplishing the heating and bending operations involves use of a thin heated rod about which the polyvinyl chloride sheet is bent. However, polyvinyl chloride sheets of the required thickness have a great tendency to warp when heated above about approximately 130.degree. F. Consequently, warpage of floppy disc envelopes in excess of permissible specifications has been frequent. Consequently, manufacturing yields have been undesirably low, increasing costs and reducing profitability.
Accordingly, it is an object of the invention to provide precise folds in polyvinyl chloride sheets while minimizing warpage therein.
Another object of the invention is to provide precise folds in polyvinyl chloride sheets without the necessity of preheating any of the polyvinyl chloride material prior to bending of the polyvinyl chloride sheets.
Yet another object of the invention is to provide a method and apparatus for folding floppy disc envelopes without causing unacceptably high warpage of the envelopes and without producing stresses which substantially warp the envelopes, increasing friction between a liner of the floppy disc envelope and an enclosed floppy disc to an unacceptable level.